1. Field
The present disclosure generally relates to the design of multi-chip modules (MCMs). More specifically, the present disclosure relates to the design of an MCM that includes a cyclic de-multiplexer in a single optical routing layer that provides full-mesh, point-to-point connectivity among chips in the MCM.
2. Related Art
Engineers are researching a multi-chip module (MCM) that integrates multiple silicon chips together into a logically contiguous piece of silicon larger than a single reticle using high-speed interconnects, such as silicon-photonic wavelength-division-multiplexing (WDM) interconnects. In order to intimately interconnect chip sites within the MCM with low latency and high bandwidth without blocking, a full-mesh, point-to-point interconnect topology with all-to-all connectivity is advantageous.
However, implementing such a fully connected mesh network using silicon optical waveguides on a silicon-on-insulator (SOI) substrate may involve a large number of optical-waveguide crossings. This is illustrated in FIG. 1, which presents a drawing of an existing 8-node, fully connected mesh network topology. In a real implementation with a larger array size, or for large communication bandwidths between sites (where each connection in FIG. 1 would represent a bundle of optical waveguides), there would be a significantly larger number of optical-waveguide crossings. Such optical-waveguide crossings introduce channel loss and crosstalk from interfering channels and are therefore desirable to avoid. Even though special crossing designs manage to reduce the channel loss and crosstalk significantly (less than 0.1 dB insertion loss and less than −40 dB crosstalk), the hundreds of optical-waveguide crossings in an MCM would still significantly impact performance and, therefore, could pose an obstacle to the use of this technology.
One approach to eliminating optical-waveguide crossings is to use surface normal optical proximity couplers (OPxCs) and dual-layer routing. This approach can facilitate scalable full-mesh networks in WDM silicon-photonic links. In such designs, there are typically four OPxC hops in each WDM link. Currently, each OPxC hop introduces a loss of about 3 dB. As a consequence, currently four OPxC hops will add a total of 12 dB to the link loss, which results in large link-loss budget and significantly limits the energy efficiency of the WDM links and the interconnect network as a whole. While there are ongoing improvements in OPxCs, the losses associated with OPxC hops are expected to remain significant. As a consequence, it is desirable to reduce the number of OPxC hops and, thus, the number of routing layers needed to complete the logical connectivity required by the fully connected mesh network.
Hence, what is needed is an MCM that does not suffer from the above-described problems.